Cadmium when given to several different mammalian species in relatively low dosages causes specific damage to the blood vascular system of the testes. Why this vascular bed is so sensitive to cadmium remains an intriguing and yet unanswered question. The purpose of this study is to determine how cadmium effects the ultrastructure of this area under a variety of test situations and what unique characteristics exist here which may make these vessels especially sensitive to cadmium. Four experiments will be conducted, each concerned with a specific problem or aspect of the effect of cadmium upon the testis as follows: Experiment 1 will attempt to determine where and how zinc and selenium protect the vascular system of the testes from the damaging effects of cadmium. Experiment 2 outlines an attempt to localize cadmium in the testes and determine changes in the proportion of other cations by using an energy dispersive x-ray analyser. Experiment 3 will define the role that temperature changes play in the effect of cadmium on the testes. Experiment 4 describes ultrastructural examination of cadmium-resistant revascularized testes and long term cryptorchid testes after exposure to cadmium. Finding the reason for the sensitivity of the vasculature of the testis to cadmium would contribute valuable information about the basic functioning of the testis and would contribute to subsequent research on contraception and fertility.